For formation of a ceramic-based honeycomb structure, there has been widely used a method of conducting extrusion by the use of a die for formation of honeycomb structure, having a die base comprising back holes for introducing a forming material (a puddle) thereinto and slits (e.g. slits having a lattice shape) communicating with the back holes.
In the die, there are provided, at one face of the die base, slits of, for example, lattice shape, each having a width corresponding to the partition wall thickness of a honeycomb structure to be formed with the die and, at the opposite face (the other face), back holes communicating with the slits. The back holes are provided ordinarily so as to correspond to the positions at which the slits of lattice or other shape intersect each other, and the slits and the back holes communicate with each other inside the die base. Accordingly, a forming material (e.g. a ceramic material) introduced from the back holes moves from the back holes of relatively large inner diameter into the slits of small width, and is extruded from the opening portion of each slit as a honeycomb formed body.
In one method for manufacturing the die base, a metal block is subjected to drilling or the like at its one face to form a plurality of back holes and to grinding, electric discharge machining or the like at the other face to form slits communicating with the back holes. In this case, when the die base is for formation of a honeycomb structure having a square cell section, slits are formed along the contour of the square that is, the slits intersect each other at right angles and, when the die base is for formation of a honeycomb structure having a hexagonal cell section, slits are formed along the contour of the hexagon; the crossing portion of slits are allowed to correspond to the back holes.
As the die base constituting the die to be used for formation of honeycomb structure, there are used, for example, a plate-shaped member (a die precursor) made of one kind of alloy (e.g. stainless steel alloy or super hard alloy) and a plate-shaped member (a die precursor) obtained by bonding two different kinds of plate-shaped members (see, for example, Patent Literatures 1 and 2).
First, the die base made of stainless steel or the like is low in abrasion resistance and, therefore, has a problem that the slits wear in continuous extrusion and the shape of honeycomb formed body extruded changes gradually; in order to prevent it, there is ordinarily formed, on the surface of the die base, a coating layer [e.g. a plating layer (an electrolytic plating] or a titanium-based film (CVD or PVD)].
In the die base provided with a coating layer, appropriately curved surface parts (R shape parts) are formed at each slit intersection and, moreover, the coating layer wears gradually and slowly; as a result, the honeycomb formed body extruded can maintain an intended shape.
When the coating layer has worn to a level higher than specified, the die base is replaced with a new die base body; the old die base body is subjected to thorough complete removal of coating layer; the resulting die base body is provided with a fresh coating layer and then subjected to pattern adjustment; thus, regeneration of die base body is conducted. However, the regeneration of die base body requires much labor and time and further there is a limit in the times of regeneration; thus, the regeneration of die base body has had a problem of high cost.
Meanwhile, the die base made of a super hard alloy is superior in abrasion resistance and, therefore, is strikingly low in slit abrasion; accordingly, this die base body can be used over a long period of time with substantially no maintenance.
However, when slits have been formed in the die base made of a super hard alloy by grinding or electric discharge machining, each intersection of slits has no appropriately curved surface part (no R shape part) at the corners and has, as shown in FIG. 12(b), corners 30 of acute angle in slit crossing portion D; consequently, there has been, in extrusion of honeycomb formed body, a problem of inferior formability (e.g. occurrence of partition wall breakage) or inferior properties (e.g. low isostatic strength) of honeycomb formed body.
Further, being a fragile material, the super hard alloy has had a risk that the die made thereof undergoes, during extrusion, concentrated stress at the corners of slit intersection, resulting in breakage of die. Furthermore, when a coating layer is formed on the die base made of a super hard alloy as in the case of the die base made of stainless steel or the like, the formation of coating layer is conducted mainly by electrolytic plating; in case of using super hard alloy, the die base is deteriorated by an acid or alkali used in a plating solution or in a cleaning step and a fragile layer is formed on the surface of super hard alloy; resultantly, the die base is deteriorated in abrasion resistance and no merits attributed to the use of super hard alloy has been obtained. Even if the formation of coating layer is made by other means, much labor and time are required, inviting a high cost; thus, there has been a problem that there is no advantage of using a die base made of a super hard alloy.
In order to solve the above problems, there is, for example, a method of forming, by electrolytic machining, a R art at each corner of slit intersection of die base body (see, for example, Patent Literature 3). However, electrolytic machining apparatus, regardless of the kind of electrolytic solution used therein, has not spread widely among ordinary die manufactures, as compared with electric discharge machining apparatus, and the wide use of the above method is difficult. Further, when the slit portion of die base body is made of a super hard alloy, such a super hard alloy contains tungsten carbide and Co as a binder and, in particular, tungsten carbide is hardly ionized; therefore, it is easily considered that tungsten carbide is accumulated, during electrolytic machining, between the electrode and the slits as chips, making intended electrolytic machining impossible. Furthermore, since there is used, as the electrolytic solution, a strong alkali or a strong acid, the super hard alloy portion is deteriorated and the handling of electrolytic machining apparatus is dangerous, requiring a very high cost for the safety measure, etc. of facility; thus, the wide use of the above method is difficult. Meanwhile, in electric discharge machining, an oil is usable as the electric discharge machining solution, the handling thereof is easy, and the corrosion of super hard alloy is minimized; therefore, it is being investigated to form an appropriately curved surface part (reverse R part or circle) in each rib of discharging electrode (rib electrode for electric discharge machining) in order to transcribe that part on each slit intersection of the die base made of a super hard alloy. However, this has not been realized because the number of the above-mentioned rib of discharging electrode (rib electrode for electric discharge machining) is many (20,000, for example) and, moreover, the electrode per se is exhausted by the joule heat generated during electric discharge machining, unlike in electrolytic machining.    Patent Literature 1: JP-A-2000-326318    Patent Literature 2: JP-A-2003-285308    Patent Literature 3: JP-3080563